Pressure Swing Adsorption (PSA) is a technique used to fractionate mixtures of gases to provide at least one purified product gas and a raffinate byproduct mixture. PSA has been successfully used to separate hydrogen from other gases, oxygen and nitrogen from air, and helium from natural gas, among others.
PSA systems may include multiple vessels containing adsorbents to separate the gases. The vessels may include upper and lower heads through which flow conduits or nozzles are disposed.
During the PSA process, adsorption vessels undergo various stages of a cyclic process including feed, pressure equalization, product pressurization, counter-current blowdown, and purge, among others. Flow during the cyclic process includes flow of gases that traverses upward through the adsorbent bed(s) as well as downward through the adsorbent bed(s). Flow during the cyclic process also goes through rapid pressure and/or temperature swings; for example, flow at a vessel nozzle may be high pressure in one direction followed by low pressure in the opposite direction.
To prevent loss of the adsorbent through the nozzle(s) in the upper and lower heads of the adsorption vessel, screens or other devices have been used. One example of a prior screen includes a system of welded metal bars arranged in a “basket” shape. As another example, steel plates welded to the pressure shell in a conical or cylindrical shape with perforations or passages cut to permit flow. Additionally, trays covered by wire mesh with a large dead space below the tray have been used. These and other various designs may impart stresses to the adsorption vessel, may erode or corrode, such as due to liquids accumulation and imperfections due to welding, among others. In some instances, the stiff flow distribution apparatus have created high localized bending stresses, which have resulted in cracking of the adsorption vessel. This problem of bending restraint is particularly-pronounced in systems processing embrittling fluids, such as mixtures containing hydrogen, hydrogen sulfide, hydrogen cyanide, and ammonia, among others. Further, the various designs may be impractical or difficult to replace once the vessel is installed in the field. This further limitation is particularly-deleterious in designs which lack compressive strength to deal with high momentary pressure gradients, as may occur due to clogged fine screens within the distributor, clogged beaded packed bed media adjacent to the distributor, to abrupt failure of valves or piping attached to the vessel, or to combinations of these causes. It is particularly vexing that distribution means which are sufficiently stiff to resist collapse, such as those welded to the inner surface of the domed end cover, referred to as a head in the pressure vessel art, of the adsorption vessel are also particularly-inclined to give rise to localized bending stresses, which can give rise to cracks not only in the flow distribution apparatus, but also in the primary pressure-retaining surfaces of the pressure vessel—i.e. the heads or shell. Such cracks disadvantageously permit leaks, which may pose significant safety risks if the fluid being treated poses risks due to flammability, toxicity or asphyxiation, among other exemplary risks.